Which mechanism of arrhythmia is associated with accelerated phase 4 Na+ leakage into cardiac cells?

• A. Abnormal automaticity
• B. Reentry
• C. Triggered activity
• D. Atrioventricular dissociation

Answer: (A)

Accelerated phase 4 depolarization with increased Na+ influx in non-pacemaker cardiac cells makes them reach the threshold on their own and fire spontaneously. This creates ectopic pacemaker activity, producing extra or premature impulses that can trigger arrhythmias. That behavior matches abnormal automaticity, where cells outside the normal pacemaker system gain automaticity due to a steeper diastolic depolarization.

Reentry needs a circulating circuit, triggered activity refers to afterdepolarizations (early or delayed) tied to abnormal repolarization or Ca2+ handling, and atrioventricular dissociation is a conduction block phenomenon. The mechanism described—self-initiated impulses from accelerated phase 4 Na+ leakage—best fits abnormal automaticity.